1. Field of the Invention
The invention relates to an equipment and process used for physical refining and/or desodorisation of edible oils and fats.
More particularly, the invention relates to a process for vacuum stripping triglyceride oils and fats.
More specifically, the invention is also concerned with novel equipment used in this process.
2. Discussion of the Related Art
The invention is especially important for edible oils and fats to be sold for consumption or to be used in food products, since these outlets demand that these edible oils and/or fats have a bland taste. In addition, the invention is applicable to all kinds of edible oils and fats such as vegetable oils, animal fats and marine oils, their blends as well as to hydrogenated oils, fractionated oils, interesterification products and their blends. It will in general constitute the last step in edible oil processing before the product treated according to the invention is either packaged, or further processed into products such as for instance margarine.
In edible oil refining, two different routes are being employed. The oldest route employs a chemical neutralisation of the free fatty acids in the crude oil as obtained by expelling and/or solvent extraction. This route commonly uses caustic soda to convert these free fatty acids into sodium soaps which are then removed by a centrifugal separator and water washing. Instead of removing residual soaps by water washing, they can also be removed by adsorption by for example silica hydrogel.
Subsequently, the neutral oil is bleached by an adsorptive treatment involving the use of bleaching earth and/or activated carbon and then the malodorous compounds still present in the oil are removed by a vacuum stripping process called deodorization.
In the other route, the crude oil is first of all degummed by using an acid degumming or acid refining process to a low residual phosphorus content <10 ppm P) without removing the free fatty acids. If such a degummed oils is then bleached it can be physically refined in a vacuum stripping process that removes both the free fatty acids and the malodorous compounds so that a bland tasting product of good keepability results. Physical refining therefore combines the neutralisation and the deodorization of the oil.
The process according to the invention can be usefully applied in both the deodorization process and the physical refining process.
In the deodorization and physical refining processes already known, it is common to subject the oil to be processed to temperatures as high as 250° C. or even higher, a pressure as low as 5 mbar or even lower and to the action of a stripping medium. The commonly used stripping medium is steam but other inert gases such as for example nitrogen are also being used as described in EP-A-0.580.896 to Cheng et al. In this context, the role of the stripping medium is to dilute the volatile compounds to be removed from the oil so that they can evaporate at higher pressures than if no stripping medium were present.
For deodorization and physical refining to be effective and to minimise the stripping medium usage, the concentration of the volatile compounds to be removed from the oil should be as close as possible to the physical equilibrium concentration as determined by the vapour pressures of the pure volatile compounds at the prevailing temperature and their concentrations in the oil. To this end, intimate contact between the stripping medium and the oil to be vacuum stripped is imperative. Such contact is achieved by introducing the stripping medium under the surface of a pool of liquid oil to be deodorised through small nozzles or by spreading this oil over a large surface that is in contact with the stripping medium. The latter method as described for instance in WO 98/00484 to Hillström et al. commonly employs packed columns for this purpose.
However, when a small bubble of stripping medium is released under the surface of a pool of oil, the pressure at the point of release is equal to the sum of the system pressure and the pressure exerted by the oil column above the release point. When the bubble rises through the oil, the height of this column decreases so that the pressure inside the bubble also decreases as a result of which the bubble will expand. At the surface, the bubble will break and the gas contained in the bubble will be removed by the vacuum system. However, in doing so, the bubble will also entrain liquid oil and if this oil were to reach the vacuum system, it would be irretrievably lost and thus would constitute a yield loss. This loss is often referred to as the “neutral oil loss” or NOL.
Accordingly, most industrial deodorisers have been provided with baffles that aim at retaining the oil that is being entrained by the emerging stripping medium and thus at reducing the NOL. However, these baffles often constitute a resistance to the vapour flow and thus cause the pressure at the point where the bubble leaves the oil surface to be considerably higher than the vacuum attained by the vacuum system itself. Packed columns suffer less from NOL since in these columns the stripping medium strikes over the oil surface but then, these columns themselves present a resistance to the vapour flow so that again, the pressure “seen” by the oil can be quite a bit higher than the pressure attained by the vacuum system.
As mentioned before, oil to be deodorised or physically refined is brought to elevated temperatures since the vapour pressures of the volatile compounds to be removed by stripping increase with an increase in temperature so that their volatility increases. In order to save energy, the incoming oil is often pre-heated by the outgoing oil by an oil-to-oil heat recovery process so that external energy is only needed to raise the oil temperature from the level attained by heat exchange to the deodorization temperature. The heat exchange system and/or the final heating system can located outside the deodorization vessel proper but most installations in use incorporate these systems within the deodorization shell. The de-aeration stage on the other hand, tends to be outside this shell.
Accordingly, such a shell usually contains a number of superimposed trays from which the oil to be deodorised flows downwards by gravity. From top to bottom these trays may serve the following functions: de-aeration if this is contained within the shell, heating by heat exchange with outgoing oil, heating to final deodorization temperature, deodorization proper, cooling by heat exchange with incoming oil and finally further cooling to oil discharge temperature.
In general, all trays are sparged with stripping medium which not only strips volatiles from the oil being processed, but also ensures agitation of the oil and thus promotes heat transfer. Deodorization proper need not be confined to a single tray but may be carried out in a number of superimposed trays. Similarly, the oils can be heated or cooled after having been subjected to the first deodorization treatment before being subjected to a subsequent deodorization treatment.
If the trays within the deodoriser shell are fixed to the deodoriser shell wall and surround a central chimney which collects the vapour emerging from the trays and mechanically supports these trays, the deodoriser is generally referred to as a single shell deodoriser. Instead of a central chimney, the gases can also be removed from each tray by individual vacuum connections connected to a common duct. If an annular gap between the trays and the outer shell acts as the vapour duct, the system is referred to as a double shell deodoriser.
Flow diagrams of the various types, double shell, single shell, continuous and semi-continuous can be found in the Practical Handbook of Soybean Processing and Utilization, edited by D. R. Erickson, AOCS Press and United Soybean Board, 1995 on pages 251 and following. In general, a single shell deodoriser is less expensive to construct than a double shell deodoriser.
In order to maintain a low pressure within the deodorization shell, it has to be connected to a vacuum system. This system removes the stripping medium that is introduced into the shell, the volatiles entrained by this medium and any gas leaking into the shell. These volatiles have to be removed from the gas leaving the deodorization shell and to this end, the gas is commonly passed through a scrubber. In this scrubber, the hot gas is contacted with cooled distillate into which the volatiles will condense. Accordingly, the vacuum system will not have to transport these volatiles and can thus have a smaller capacity which is especially important in the case of physical refining.
This scrubber can be located in the top of the deodoriser shell and below the connection to the vacuum system.
Liquified distillate is collected at the bottom of this scrubber and then it is partially recycled to the top of the scrubber via a cooler; what is not recycled is collected in an intermediate distillate storage vessel. However, this set-up implies that the low temperature scrubber section is immediately above a hot oil tray and this puts high demands upon the construction and construction material of the deodoriser which will have to withstand considerable thermal and mechanical stress in addition to remaining vacuum tight.
Consequently, the scrubber can also be located outside the deodorization shell in the duct that connects the deodoriser to the vacuum system. If this duct is located at the top of the deodoriser shell, the scrubber is usually nearby to minimise the pressure drop in the vacuum duct. If the vapours are collected from each individual tray into a common duct, the scrubber will be located close to this common duct but that can be at the top of this duct, at the bottom of this duct or somewhere in between.
For the generation of the vacuum, steam ejectors and condensers are commonly used. Since these require barometric legs, the vacuum system requires at least 10 m height. Consequently, commonly used deodorisers display a connection to the vacuum system at the top of the vacuum shell for instance just above the scrubber if this is located within the deodoriser shell. Since gas needs a pressure difference to flow, this means that the system pressure in the lowest part inside the deodoriser shell is somewhat higher than at the top. Accordingly, the oil being deodorised is exposed to a higher pressure than the oil being heated or degassed since the heating and deaeration trays are situated above the deodorization trays. This is a drawback of this type of construction since preferably, deodorization should be carried out at the lowest pressure the vacuum system can generate.